UNIVERSITY  OF  CALIFORNIA   PUBLICATIONS 

IN 

AGRICULTURAL    SCIENCES 

Vol.  2,  No.  6,  pp.  205-216,  plates  39-41  November  23,  1920 


INBREEDING  AND  CROSSBREEDING  IN 
CREPIS  CAPILLARIS  (L.)  WALLR. 


BY 

JULIUS  L.  COLLINS 


INTRODUCTION  AND  BRIEF  DISCUSSION  OF 
INBREEDING  EFFECTS 
It  is  well  established  that  continued  inbreeding  within  a  strain 
or  race  may  produce  results  harmful  to  individuals  of  that  race.  It 
is  only  in  modern  times,  however,  that  a  consistent  explanation  of 
the  causes  of  such  results  has  been  made.  This  explanation  of  the 
problem  has  come  through  carefully  planned  and  executed  experi- 
ments upon  plants  and  animals  and  through  a  recognition  of  the 
Mendelian  laws  of  heredity.  The  most  extensive  and  comprehensive 
of  these  investigations  is  that  with  maize,  started  by  East1  in  UUo 
and  now  being  carried  on  by  Jones,  at  the  Connecticut  Agricultural 

Experiment  Station.  . 

Inbreeding  is  now  considered  and  used  as  a  method  by  which  the 
hereditary  constitution  of  the  germ-plasm  can  be  made  evident.  In- 
breeding, as  such,  produces  no  evil  results.  The  abnormal  forms  that 
sometimes  appear  in  inbred  strains  show  up  because  the  recessive  genes 
conditioning  such  forms  are  present  in  the  germ-plasm.  If  no  such 
genes  are  present,  no  amount  of  inbreeding  can  produce  them. 

The  fact  that  inbreeding  produces  abnormal  forms  and  reduction 
of  vigor  in  some  species  and  not  in  others  is  due  to  a  condition  of 
the  germ-plasm.  For  example,  no  such  results  attend  inbreeding  in 
self-fertilized  crops  like  wheat  and  barley  because  in  them  self-fertil- 
ization is  the  normal  method  of  reproduction  and  such  plants  are 
homozygous  for  all  their  genes,  all  the  abnormal  and  weak  plants 
"  ,  Easti  E.  M..  and  Jones,  D.  F.,  Inbreeding  and  Outbreeding,  pp.  1-285,  46 
illus.,  Philadelphia,  Lippincott.     1919. 


206  University  of  California  Publications  in  Agricultural  Sciences        [Vol.2 

having  long  ago  been  segregated  out  of  the  race  and  having  perished 
in  the  competition  with  their  more  hardy  and  vigorous  sibs. 

Maize  is  a  naturally  cross-fertilized  species,  and  heterozygosity  is 
therefore  the  general  condition  of  the  germinal  material  instead  of 
homozygosity,  as  is  the  case  in  self-fertilized  species.  In  this  hetero- 
zygous condition  the  genes  of  recessive  harmful  characters  may  be 
carried  along  in  the  germ-plasm  under  the  protection  of  desirable 
dominant  characters  and  appear  only  when  the  latter  are  absent. 
Inbreeding  furnishes  conditions  favorable  to  the  accumulation  of  these 
recessive  genes  in  the  germ-plasm  and  for  the  appearance  of  the 
recessive  characters  in  some  of  the  individuals. 

The  increase  in  size  and  vigor  observed  in  the  progeny  when  two 
different  inbred  strains,  or  inbred  strains  and  unrelated  non-inbred 
strains,  are  crossed  is  due  to  the  establishmeiit  of  a  heterozygous  germ- 
plasm  containiii"-  more  dominant  factors  influencing  size  and  vigor 
than  were  present  in  either  of  the  parents.  Linkage  of  such  genes 
to  recessive  or  to  dominant  genes  which  influence  vigor  and  size  ad- 
versely prevents  the  production  of  homozygous  dominant  races.  For 
this  reason  the  vigor  noticed  in  the  Fx  is  less  marked  in  the  F2  and 
subsequent  generations  where  segregation  and  recombination  have 
taken  place. 

Most  of  the  knowledge  of  the  effects  of  continued  inbreeding  and 
the  results  obtained  from  crossing  inbred  strains  have  come  from 
experiments  on  plants  and  animals  under  domestication.  Such  species 
have  been  the  subject  of  conscious  selection  for  particular  types,  which 
often  preserves  in  the  species  characters  desirable  from  an  agricul- 
tural point  of  view,  but  so  detrimental  that  the  race  could  not  exist 
except  under  the  conditions  of  domestication.  It  has  been  questioned 
whether  the  germinal  material  of  such  races  is  comparable  to  that 
of  wild  species  in  which  natural  selection  may  have  largely  eliminated 
from  the  germ-plasm  genes  which  produce  characters  detrimental  to 
the  natural  existence  of  the  species.  In  view  of  this  possibility  the 
question  has  arisen  as  to  whether  or  not  the  results  of  continued 
inbreeding  would  be  the  same  if  wild  species,  in  place  of  domesticated 
ones,  were  the  subject  of  such  experimentation.  It  is  in  this  connection 
that  this  report  on  inbreeding  in  Crepis  is  of  interest. 


1920]  Collins:   Inbreeding  and  Crossbreeding   in   Crepis  Capittaris  207 


MATERIAL  AND  METHODS 

Crepis  capittaris  is  a  species  belonging  to  the  chicory  tribe  of  the 
Compositae.  The  species  is  a  native  weed  of  European  and  North 
African  countries,  and  has  been  introduced  into  both  North  and  South 
America,  where  it  grows  in  limited  localities  as  a  common  weed.  It 
is  either  annual  or  biennial.  The  flowers  are  all  perfect  and  both 
cross  and  self-fertilization  take  place  under  natural  conditions.  In 
nature  it  is  quite  variable  in  a  number  of  ways  according  to  the 
environmental  conditions  in  which  it  grows,  but  our  breeding  experi- 
ments show  that,  when  grown  continuously  under  the  same  conditions, 
constant  forms  are  produced  in  successive  generations.  No  records 
have  been  found  of  its  subjection  to  extensive  artificial  selection  and 
it  is  therefore,  in  the  true  meaning  of  the  word,  a  wild  species. 

In  order  to  reduce  the  effect  of  variation  in  the  environmental 
factors  of  soil,  light,  temperature,  moisture,  and  space  to  the  minimum 
care  was  taken  to  have  the  soil  homogeneous,  to  have  the  same  size 
and  kind  of  pots,  and  to  grow  successive  generations  of  plants  in  the 
same  portion  of  the  greenhouse  as  their  parents  had  occupied.  This 
last  item  was  varied  in  the  last  generation  (1920)  to  the  extent  of 
placing  both  inbred  and  hybrid  cultures  on  a  bench  on  the  opposite 
side  of  the  greenhouse  from  the  side  where  the  parent  cultures  grew. 
Inbred  and  hybrid  cultures  have  been  grown  side  by  side.  The  ar- 
rangement in  plate  39,  figure  1,  and  plate  40,  figure  1,  is  that- in  which 
the  plants  grew  on  the  bench.  Some  of  the  inbred  plants  and  some 
of  the  crossbred  plants  were  grown  in  both  four  and  six-inch  pots. 
This  did  not  alter  the  size  and  growth  relations  in  any  way. 

Crossing  was  accomplished  in  cultures  115  and  129  by  emascu- 
lation2 of  the  plants  intended  to  be  female  parents,  while  in  H-10  the 
water2  method  of  depollination  was  used. 


2  To  be  described  in  detail  in  another  paper. 


208  University  of  California  Publications  in  Agricultural  Sciences        [Vol. 


INBREEDING  AND  CROSSBREEDING  EXPERIMENTS 

Cultures  of  Crepis  were  first  grown  to  study  and  to  isolate  certain 
character  variations  which  had  been  observed.  Forced  inbreeding 
was  resorted  to  as  the  quickest  .means  of  reaching  the  desired  end. 
After  two  generations  of  inbreeding  it  was  noticed  that  the  plants 
were  much  smaller  and  less  hardy  than  at  first,  notwithstanding  the 
fact  that  cultural  methods  had  not  varied  to  any  marked  extent.  Ex- 
periments were  then  planned  and  executed  with  these  cultures  to 
demonstrate  the  effects  of  continued  inbreeding  and  subsequent  cross- 
ing in  a  wild  species,  and  the  results  obtained  form  the  body  of  this 
report. 

In  Table  1  are  given  the  pedigrees  of  the  cultures  in  which  in- 
breeding was  continued.  Cultures  20.113,  20.114,  and  20.128  have 
identical  ancestors  previous  to  their  parents,  which  were  sibs.  In 
the  second  generation  of  inbreeding,  sibs  of  culture  e2  were  crossed 
because  the  strain  showed  such  a  high  degree  of  self-sterility  that  it 
was  feared  that  not  enough  viable  seed  could  be  secured  to  maintain 
the  strain.  By  crossing  sibs,  which,  however,  were  very  similar  in  all 
respects,  a  few  viable  seeds  were  secured.  Self-fertilized  seeds  of 
e32P(i  and  of  e32PIS  were  also  secured  and  their  cultures  were  in  all 
measurable  respects  no  less  vigorous  than  the  progeny  of  crossed  sibs 
of  culture  e32.  Plate  41,  figure  1,  shows  culture  113,  derived  from 
crossing  sibs,  and  culture  114,  derived  from  selling  one  of  the  sibs 
used  in  the  cross.  Thus  for  inbreeding  purposes  the  culture  e2  had 
reached  an  almost  homozygous  condition  in  the  third  generation,  since 
in  no  case  have  appreciable  changes  been  noticed  in  the  fourth 
generation. 

Culture  e28  also  seemed  to  reach  its  maximum  reduction  of  vigor 
and  size  in  the  third  generation  of  inbreeding.  No  crossbred  plants 
from  this  inbred  strain  have  yet  been  grown.  In  contrast,  H-10 
(pi.  41,  fig.  2),  resulting  from  crossing  sibs  in  the  two  previous  gener- 
ations, shows  but  little  reduction  in  vigor  or  size,  indicating  that  it 
is  either  still  heterozygous  genetically  or  is  not  affected  by  inbreeding 
to  the  same  degree  as  e28.  The  latter  seems  to  the  writer  more  prob- 
able, inasmuch  as  the  entire  culture  of  H-10  was  fairly  uniform,  thus 
indicating  a  large  degree  of  homozygosity. 

Cultures  17.192  and  Z9  used  in  the  crossbreeding  experiments 
were  chosen  because  they  could  have  no  immediate  genetic  relation 


1920] 


('nil ms:   Inbreeding   and   Crossbreeding   in  Crcpis  CapUlaris 


209 


to  the  inbred  cultures.  Seed  secured  from  wild  plants  in  Berkeley, 
California,  was  used  in  1916  to  start  the  culture  17.192.  Cambridge 
(Quy  Fen),  England,  is  the  source  of  culture  Z9.  The  latter  were 
slightly  more  vigorous  than  the  Berkeley  plants.  Culture  17.178  was 
grown  from  seeds  from  wild  plants  found  growing  near  Eureka, 
Humboldt  County,  California. 

Culture  e33,  which  was  used  as  one  of  the  parents  of  crossbred 
culture  129,  is  a  reciprocal  of  e32,  and  similar  in  all  respects. 

Pedigrees  of  the  plants  used  in  these  experiments  are  shown  in  the 
accompanying  tables.  In  Table  1  two  systems  of  symbols  are  used  to 
designate  cultures.  In  the  parent  stock  and  in  the  first  and  fourth 
generations  the  annual-notebook-page-number  system  of  Shull  is  used. 
In  the  second  and  third  generations  individual  cultures  are  designated 
by  key  letters  combined  with  numbers.  In  both  tables  individual 
plant  numbers  are  designated  by  P  with  a  subscript.  In  Table  2  the 
same  systems  are  used  together  with  special  key  letters  (H  and  Z)  to 
designate  certain  cultures. 

Table  1 — Showing  Pedigrees  op  Plants  Used  in  the  Inbreeding  Experiment 


Parent 

Generations 

of  Breeding 

Stock 

First 

Second 

Third 

Fourth 

17.178P6o 

18. 1SP6 

e2P  2X16 

e32P  6X12 

20.113 

17.17SP60 

I8.I8P5 

e2P  2X16 

e32P6 

20.114 

17.178P6n 

18.18P6 

e2P  2X16 

e32P18 

20 . 128 

17.178Pi2 

18.31P9 

eSP62 

e28 

Table  2 — Pedigrees  of  Crossbred  Plants 


17.192P7X4 

— 204P,j6X27 

— mo 

17.192P7X4 

■ — 204P27X6 

— HQPS 
X 

—20.129 

17.17SP6o 

— I8.I8P5 

— e2Pl6x2 

-e33P6 

17.178Peo 

—18.  ISPs 

— e2P2xl6 

— e32P16 
X 
Z9P6 

>      —20.115 

210  University  of  California  Publications  in  Agricultural  Sciences        [Vol.  2 


DISCUSSION  OF  RESULTS 

One  would  expect  that  the  germ-plasm  of  an  old  wild  species 
had  been  largely  purified  of  the  genes  which  cause  the  production  of 
abnormal  and  harmful  characters  by  the  elimination  of  weak  forms 
through  natural  selection,  but  our  experience  with  Crepis  demon- 
strates that  such  is  not  the  case  in  a  race  partially  cross-pollinated. 
The  germinal  material  of  Crepis  capillaris  is  maintained  in  a  hetero- 
zygous condition  by  natural  cross-pollination,  as  is  the  case  in  the 
cultivated  species  of  maize.  This  is  shown  first  by  the  marked  re- 
duction in  the  size  of  the  plants  and  their  slower  rate  of  development, 
and  secondly  by  the  fact  that  we  have  isolated,  by  inbreeding  and 
selection,  constant  breeding  forms  which  differ  in  the  characters  for 
which  selected. 

The  maximum  amount  of  the  effects  of  inbreeding  appears  to 
occur  in  the  second  and  third  generations.  Forms  have  been  observed 
in  inbred  cultures  which  have  not  been  observed  in  wild  colonies ; 
no  doubt  a  more  extended  observation  would  show  that  they  do  occur, 
though  rarely.  Pollen  sterility  is  one  of  the  results  of  inbreeding 
and  one  plant  has  appeared  in  a  third  generation  culture  which  pro- 
duced almost  no  pollen.  In  the  culture  produced  by  growing  seed 
of  wild  plants  which  themselves  grew  in  New  Zealand  we  have  also 
found  one  plant  (N.  Z.  P7,  1920)  which  produces  no  pollen  at  all; 
other  plants  of  this  culture  appear  normal  in  pollen  production. 
This  is  evidence  that  this  character  may  also  appear  in  wild  plants. 

Strains  of  fasciated  plants  have  been  isolated  in  Crepis  tectorum 
which  are  so  weak  that  it  is  only  by  starting  a  large  number  that  we 
can  get  a  few  to  live  long  enough  to  produce  seed,  yet  the  plants  in 
the  heterozygous  condition  seem  to  be  in  no  way  affected. 

Most  of  the  plants  were  grown  in  four-inch  clay  greenhouse  pots. 
In  order  to  determine  whether  this  limiting  of  the  root  space  would 
in  any  way  accentuate  the  dwarfishness  of  the  inbred  plants,  part  of 
inbred  strain  No.  128  and  part  of  hybrid  culture  No.  129  were  grown 
in  both  four-inch  and  six-inch  clay  pots  and  placed  in  adjacent  rows 
on  the  bench.  Plate  40  showing  plants  in  six-inch  pots  and  plate  39 
showing  similar  cultures  in  four-inch  pots  answer  this  question  in  a 
very  definite  manner. 


1920]  Collins:   Inbreeding   and  Crossbreeding   in   Crepis   Capillari.i  211 

The  results  of  inbreeding  in  Crepis  support  the  statement  of  East 
and  Jones3  that  in  naturally  cross-fertilized  organisms  the  immediate 
results  of  inbreeding  are  most  emphatically  injurious,  but  it  must  be 
considered  as  an  exception  to  their  statement  that  "wild  types,  in 
general,  might  not  present  such  an  appearance  of  injury  under  in- 
breeding as  shown  by  cultivated  species."  Maize  is  characterized  by 
the  occurrence  of  both  cross  and  self-fertilization,  and  when  this 
condition  exists  in  wild  species  we  may  expect  such  species  to  behave 
like  maize  when  subjected  to  forced  inbreeding. 


SUMMARY 

Inbreeding  in  a  naturally  cross-fertilized  wild  plant,  Crepis  capil- 
laris,  causes  conditions  in  many  ways  similar  to  the  conditions  produced 
by  inbreeding  in  maize. 

The  maximum  reduction  appears  to  be  reached  in  the  third  and 
fourth  generations. 

Crossing  inbred  strains  with  non-inbred  strains  produces  vigorous, 
rapidly  growing  Y1  plants. 

Inbred  plants,  when  compared  with  crossbred  plants,  show  a  slower 
rate  of  development  during  the  entire  period  of  growth. 

Some  of  the  inbred  strains  showed  pollen  sterility  by  a  reduction 
in  the  number  of  mature  pollen  grains  formed. 

Increased  size  of  pots  and  quantity  of  soil  did  not  affect  the 
relationship  of  vigor  and  of  growth. 

The  results  of  the  experiments  on  Crepis  indicate  that  the  results 
of  inbreeding  maize  as  reported  by  East  and  Jones4  and  others  are 
in  no  way  peculiar  to  that  species,  but  may  be  found  to  hold  for  other 
species,  either  domesticated  or  wild,  when  similar  conditions  affecting 
sexual  reproduction  obtain. 


3  Op.  tit. 

4  Op.  cit. 


PLATE  39 
Crcpi.s  capillaris 

Fig.  1.  At  right  and  left  are  plants  representing  the  fourth  generation  of 
in  1  needing  in  two  related  strains. 

The  central  plant  is  the  result  of  crossing  an  inbred  plant  of  the  third 
generation  with  a  totally  unrelated  non-inbred  strain  of  Crepis. 

Fig.  2.  The  same  three  plants  as  shown  in  figure  1,  photographed  about 
six  weeks  later,  showing  the  precociousness  of  the  hybrid  (115)  when  compared 
with  the  inbred  plants. 

Plants  growing  in  four-inch  pots. 


[212] 


UNIV.    CALIF.    PUBL.    AGR.    SCI.    VOL.    2 


COLLINS  I     PLATE    39 


Fig.   1 


Fig.  2 


Digitized  by  the  Internet  Archive 

in  2012  with  funding  from 

University  of  California,  Davis  Libraries 


http://archive.org/details/inbreedingcrossb26coll 


PLATE  40 
Crepis  capillaris 

Fig.  1.  20.129/4.  Hybrid  secured  by  crossing  an  inbred  plant  with  a  non- 
related  non-inbred  plant. 

20.128/10.  Inbred  plant  of  the  fourth  generation;  progeny  of  the  inbred 
parent  of  the  hybrid  plant  129P4. 

Fig.  2.  The  same  two  plants  photographed  about  six  weeks  later  showing 
marked  vigor  of  the  hybrid. 

Growing  in  six-inch  pots. 


[214] 


UNIV.    CALIF.    PUBL.    AGR.    SCI.    VOL.    2 


[COLLINS]     PLATE    40 


f 


Fie.  1 


Fig.  2 


PLATE  41 
Crepis  capillaris 

Fig.  1.  Cultures  113  and  114  are  inbred  plants  of  the  fourth  generation. 
Culture  115,  Fj  hybrid  plants  of  the  same  age  as  the  cultures  113  and  114. 

Fig.  2.  e28/25  and  e28/21,  inbred  plants.  H10/11.  A  non-related  plant 
produced  by  continual  crossing  of  sibs.     Growing  in  four-inch  pots. 


[216] 


UNIV.    CALIF.    PUBL.    AGR.    SCI.    VOL.    2 


[  COLLINS  ]     PLATE    41 


Fig.  1 


I 


Fig. 


